Andy, thanks for insight.

Found the smaller MC at Advance last night and swapped in out. Loosened the rear shoes a bit while at it.

Went out and warmed up existing pads. Not much difference.

Tested prop valve and started to get bias with valve all the way out. I can notice the front begin to engage now as I get a bit of nose down went applying brakes at speed.

Once the brakes seemed as good as they'd get I did a few panic stops (15-20mph) and still get rear lock up.
Not as severe now, though.

Will wait for loaded Diplomat Calipers to arrive.

Any pad recommendation for those? May need to source a better pad.